Engines may be configured with boosting devices, such as turbochargers or superchargers, to increase airflow into a combustion chamber. Turbochargers and superchargers compress intake air entering the engine using an intake compressor. While a turbocharger includes a compressor that is mechanically driven by an exhaust turbine, an electric supercharger includes a compressor that is electrically driven by a motor. In some engine systems, one or more intake charging devices may be staged in series or parallel in what may be referred to as a compound boosting configuration. For example, a fast, auxiliary boosting device (e.g., the electric supercharger) may be utilized to increase the transient performance of a slower, primary boosting device (e.g., the turbocharger). In such a configuration, the turbocharger may be upsized to increase peak power and torque performance of the engine, which enables more aggressively downsized engines.
Various approaches may be used to provide boost control in a compound boosting system. One example approach for compound boosting system control using pressure ratios is shown by Petrovic et al. in EP 1,927,739 A1. The pressure ratio may represent the boosting capability of a boosting device of the compound boosting system. In the approach of Petrovic, a method for coordinating two turbochargers based on desired partial pressure ratios is disclosed. Specifically, the desired partial pressure ratios for each turbocharger are determined based on calibrated look-up tables using engine speed and engine torque as inputs. The desired partial pressure ratios are then achieved through at least one of adjusting a turbocharger wastegate opening, adjusting turbine vane geometry (e.g., if a variable geometry turbine is included), and adjusting openings of turbine and/or compressor bypasses.
However, the inventors herein have recognized potential issues with such systems. As one example, it is a static approach that uses predefined calibrations to determine the desired partial pressure ratios, which may be independent of one another (e.g., the desired partial pressure ratio of one compression device does not influence the desired partial pressure ratio of the other compression device). If the approach of Petrovic were applied to a compound boosting system including an electric supercharger staged alongside a turbocharger, the approach may cause the supercharger to be run for a longer than required duration, resulting in a drop in fuel economy. As another example, the pressure ratios commanded to each compression device may be calibrated conservatively in order to minimize boost pressure overshoot. However, this can result in a slower boost response. Further still, if any of the compression devices are configured with electric assist (such as electric assist from an electric motor/generator coupled to a supercharger compressor, a turbocharger compressor, or a turbocharger shaft), opening of an exhaust waste-gate responsive to the boost pressure overshoot may represent a lost opportunity for energy recuperation at the electric motor/generator.
In one example, the issues described above may be addressed by a method comprising: adjusting operation of a first intake compression device based on a target boost pressure; commanding positive torque from an electric motor to a second intake compression device when a difference between a throttle inlet pressure and the target boost pressure is higher than a threshold; and commanding negative torque from the electric motor to the second intake compression device when the difference is smaller than the threshold. In this way, a target boost pressure can be reached faster while increasing the energy recuperation opportunity of the electric motor.
As one example, a compound boosting system may include an upstream, faster-acting, auxiliary compressor configured with electric assistance (e.g., an electric supercharger compressor) and a downstream, slower-acting, primary compressor (e.g., a turbocharger compressor). Responsive to an operator torque demand, an engine controller may dynamically allocate pressure ratios to each compressor to provide a target boost pressure. In particular, an overall pressure ratio command may be aggressively generated for the turbocharger. The overall pressure ratio command may include corresponding adjustments to an opening of an exhaust waste-gate valve coupled in a waste-gate across the turbocharger turbine. For example, as the torque demand increases, the waste-gate opening may be decreased more aggressively to direct more exhaust flow through the turbine, spinning up the turbine to spin up the turbocharger compressor. However, due to the inherently slower response time of the turbocharger, there may be a temporary shortfall in boost availability, as indicated by a higher than threshold difference between the target boost pressure and a throttle inlet pressure (or the outlet pressure of the turbocharger compressor). The shortfall may be addressed by spinning up the electric supercharger using positive torque from an electric motor. When the shortfall decreases, such as when there is a lower than threshold difference between the target boost pressure and the throttle inlet pressure, but before the throttle inlet pressure reaches the target boost pressure, the electric assistance may be disabled and the motor may be used as a generator to absorb torque from the compressor. By adjusting a timing of when the electric assistance is disabled, the throttle inlet pressure may be coasted towards the target boost pressure without incurring any overshoots, while at the same time, energy can be recuperated at the electric motor. It will be appreciated that while the example is described with reference to an electric supercharger, it is not meant to be limiting, and the same approach may be used for an alternate compression device configured with electric assistance, such as an electric turbocharger.
In this way, by more aggressively calibrating a pressure ratio commanded to each of a higher frequency auxiliary compressor configured with electric assistance as well as a lower frequency primary compressor, a target boost pressure may be achieved more efficiently. By commanding a positive speed output to an electric motor providing the electric assistance when the actual boost pressure is further from the target boost pressure, a transient boost response may be improved. The technical effect of commanding a negative speed output to the electric motor when the actual boost pressure is closer to, but not yet at, the target boost pressure, is to reduce boost overshoot, while maximizing the energy recuperation ability of the electric motor. Overall, boost pressure can be provided rapidly and more efficiently.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.